This section describes how to tunnel IPv4 packets through IPv6 networks and IPv6 packets through IPv4 networks. For instance, to pass IPv6 packets through the IPv4 network, the IPv6 packet is encapsulated into an IPv4 packet at the ingress side of a tunnel. When the encapsulated packet arrives at the egress of the tunnel, the IPv4 packet is de-capsulated.
Tunnels can be either automatic or manually configured. A configured tunnel determines the endpoint addresses by configuration information on the encapsulating node. An automatic tunnel determines the IPv4 endpoints from the address of the embedded IPv6 datagram. IPv4 multicast tunneling determines the endpoints through Neighbor Discovery.